The deep injection of wastes of various types into deeply-buried rock formations is a relatively recent field. This approach has been suggested for use with radioactive and other types of toxic wastes. For example, U.S. Pat. No. 5,310,285 (Northcott) relates to the injection burial of radioactive and other wastes of varying toxicity. The principal advantage of this technique is the potential for stable retention of wastes within a deeply-buried formation over a geological time span.
In general terms, the process involves the preparation of a water-based slurry within surface-based equipment and pumping the slurry into a well that extends relatively deep underground into a receiving stratum. The basic steps in the process include the identification of an appropriate site for the injection, preparing an appropriate well, formulation of the slurry, performing the injection operations, and capping the well. Preferably, monitoring is conducted during and after the injection to assess the slurry injection conditions and the conditions of the surrounding rock formations.
An appropriate target stratum is characterized by pores, fractures or the like. Fractures may also be created within the formation by the injection of wastes under pressure. This approach is taken in U.S. Pat. No. 5,314,265 (Perkins et al.). Alternatively, a target strata may be selected that contains existing fractures or pores, as described in U.S. Pat. No. 5,489,740 (Fletcher). As well, it is desirable that the target zone be depleted of hydrocarbons or other potentially valuable products, to prevent later intrusions into the site.
Selection of an appropriate permeable injection stratum leads to rapid bleed-off of fluids, so that the presence of a carrying agent is eliminated and the solids cannot travel far. Furthermore, once high pressure injection ceases, the solids become permanently entombed within the target stratum by the imposition of the great weight of the overlying strata. The choice of a site with a high permeability horizontal flow system is intended to direct noxious or toxic carrier liquids or leachate to flow laterally, in order that within a reasonable time frame they will not mix with potable groundwater. The choice of a target stratum with adequate volume, for example a depleted oil reservoir, assures that sufficient storage volume exists to accommodate injected fluids without a regional increase in pore pressure.
The choice of a sedimentary environment which is overall relatively rich in clays means that the leachates which are gradually developed from the solid wastes are rendered more innocuous through cation exchange and adsorption of organic molecules. As an example, a site with multiple low permeability clay-rich beds overlying the target stratum will absorb substantially all noxious ions and dissolved species before long-term contact with shallower groundwater takes place.
Also, the great depth of the burial results in long flow paths for leachates, slow groundwater velocities and a relatively high cumulative exposure to absorptive minerals.
A persistent problem faced by this method is the potential for the eventual migration of the wastes out of the target area and into an aquifer or other undesired destination. This danger may be minimized in part through the selection of an appropriate geological formation to serve as the target for the wastes. However, it is still desirable that the spread of wastes underground be monitored, ideally through the use of surface monitoring techniques conducted during and after the injection process.
As well, in light of the costs of disposing of wastes by this method and the stringent regulatory environment surrounding the disposal of toxic wastes, it is desirable that monitoring of several additional variables be conducted. This serves to optimize the slurry mixture, injection pressure and rate and total injection amount. Appropriate monitoring permits an efficient injection process with an optimum amount of slurry being injected. In this way, neither an excessive amount will be injected that might spread beyond the target zone, while generally fully saturating the target zone to make efficient use of the well.
The slurry fraction injection ("SFI") technique for disposal of solid wastes in slurry form within a porous formation involves a limited hydraulic fracturing of an appropriate target rock formation, followed by injection of wastes into the formation. This method has has been previously disclosed in general terms (see M. S. Bruno et al., SPE Publication No. 29646). The process includes the step of identifying a suitable geological formation, characterized by flat lying, laterally continuous strata. The target stratum is ideally relatively porous and permeable, overlain by relatively impermeable and non-porous strata. Formation of the slurry may feature the addition of viscosifying or other agents. The slurry is injected in a series of discrete injection episodes lasting hours or days, separated by interinjection episodes. Monitoring is carried out during and after the injection process, and consists of wellhead and bottomhole pressure monitoring and surface deformation monitoring in the region surrounding the wellhead.
An object of the present invention is to provide monitoring to assess slurry absorption within the target stratum and the spread of the waste body within the target stratum.
Various types of wastes are suitable for disposal by subsurface injection. Potential candidates are wastes that do not react with the target strata, can be readily granulated and can form a slurry suspension in turbulent flow. Wastes that may be disposed of by this means include various substances that are particularly difficult to dispose of in the conventional waste stream.
An object of the invention is to provide a suitable means to dispose of a variety of wastes, including:
oily sand from petroleum industry operations, as well as waste drilling fluids and drill chips from well drilling operations and oily sand from tank-bottom clean-outs; PA1 soil contaminated with toxic materials such as PCB, heavy metals, cyanide compounds, hydrocarbons, etc.; PA1 dredging wastes; PA1 municipal sewage sludge from which the organic wastes have been largely decomposed; PA1 waste plastics, glass, and other solid materials; PA1 fly ash, clinker or other residue from combustion of wood, coal or municipal wastes; PA1 flue gas desulphurization sludges as well as recaptured particulates from smoke or emission abatement processes, whether in solid or aqueous suspension form; PA1 high solids content sludges and residues from petroleum refining, including high ash content coke, heavy oil residues and removed solids. PA1 injection of slurry at a surface pressure between 6-15 MPa; PA1 slurry injection rate of between 1.5 and 2.0 m.sup.3 /min. and 800 m.sup.3 /day, with injection being carried out for 12-14 hours/day; PA1 slurry composition with a granular solids content between 15-35% and real-time waste concentration and slurry density control to maintain density between 1100 and 1500 kg/m.sup.3 ; PA1 process 200 m.sup.3 /day of granular wastes; PA1 enhance slurry mobility with waste materials having about 10% by volume hydrocarbon content; PA1 capability to accept slop or sand as waste material. PA1 a) relative ease of handling of waste material; PA1 b) screening of granular waste material on a continuous basis; PA1 c) a real-time monitoring apparatus to monitor and record injection parameters; PA1 d) variable speed controls linked to the monitoring apparatus to control the various slurry-forming components and maintain consistent slurry quality and delivery rate; PA1 e) relatively rapid set-up and disassembly of the system; PA1 f) slurry formation equipment capable of shearing highly viscous material to increase slurry mobility and infectivity, maintaining slurry consistency within a relatively small range, and being capable of handling relatively large amounts of waste material, in the range of at least 100 m.sup.3 /day. PA1 a) identifying a generally permeable and porous target stratum, overlain by a layer of relatively low permeability strata; PA1 b) calculating the approximate total available storage volume of the target strata, based on the approximate average thickness and area of the stratum, the average porosity of the stratum and the mechanical compressibility of the formation, and the target stratum storage capacity according to the following formula: Storage Capacity=dP/(dV.times.dt) PA1 c) calculating the optimal injectivity rate for the formation according to the following formula: Injectivity=Injection Rate/(Press.sub.inj -Press.sub.fmt), where Press.sub.inj =injection pressure and Press.sub.fmt =formation pressure. PA1 c) preparing an at least partly cased well extending from the ground surface into the target stratum; PA1 d) positioning a pressure gauge at the base of the well for measuring fluid pressure within the well; PA1 e) perforating the well casing where the well passes through the target stratum; PA1 f) performing pressure fall-off and step rate tests to evaluate flow behavior and injectivity at well bottom; PA1 f) selection of a slurry having a grain size between 2 .mu.m and 5000 .mu.m, a solids concentration up to 40% by volume for grain sizes less than 150 .mu.m and up to 20% by volume for grain sizes between 150 .mu.m and 5000 .mu.m; PA1 f) injecting a slurry of waste materials in particulate form suspended in a carrier liquid into the well in a series of injection episodes separated by interinjection periods, with the injection pressure being greater than or equal to the fracture or overburden pressure and far greater than the natural water pressure in the target strata; PA1 g) measuring the well bottom pressure of the slurried wastes during each injection episode and interinjection period; and PA1 h) terminating the injection process when the target strata is generally fully saturated with slurried wastes, as determined by the volume of wastes injected and the calculated available storage volume. PA1 i) monitoring the slurry injection and emplacement by means of measurements of wellbottom pressure within the injection wells to assess formation pressure response to the waste injection, as well as permitting pressure fall-off tests and assessment of SFI and formation mechanics; PA1 ii) monitoring wellbottom hole pressure within observation wells displaced from the injection wells within about 400 meters to provide assessment of formation pressure gradients and SFI mechanics; PA1 iii) step rate injection tests conducted within the injection well, to assess fracture extension rate and formation pressure response, as well as closure stress gradient and waste containment within the formation; PA1 iv) fluid level measurements within the offset monitoring wells to assess distribution of pressure gradients within the waste emplacement zone and to provide a measurement of waste containment; PA1 v) tracer logs (temperature or radioactive tracer injection and measurement using geophysical wireline logs) within the injection well, to determine the extent of hydraulic isolation of the formation and wellbore during the injection process and an assessment of fracture orientation within the target formation; PA1 vi) monitoring of surface deformation in the region about the wellhead through collection of tiltmeter data to assess the fracture orientation and azimuth, which permits as well a reconstruction of fracture geometry, horizontal and vertical dimensions and spread of the waste body within the target formation and the rate of change of same, and deformation within the formation, as well as a further assessment of the SFI mechanics; PA1 vii) injection parameter monitoring (real time recording of injection pressures at wellhead and wellbottom, casing pressure, injection rate, injection volumes and slurry density) to permit a correlation of formation response with the SFI operating parameters; PA1 viii) material sampling of the slurry is conduced regularly and frequently to accommodate various local regulatory requirements. PA1 composition of waste material (e.g. mud/sand/slop/water ratios) PA1 daily slurry injection volumes PA1 produced sand grain size during injection PA1 fines/clay content during injection PA1 hydrocarbon content of the sand or viscosity of the muds and slops PA1 formation grain size and stress state PA1 formation geology PA1 heterogeneous effective stress and permeability distribution in the formation PA1 repeated loading and unloading of rock stresses PA1 wellbore cement quality PA1 wellbore completions quality (casing, perfing etc.). PA1 a) drilling (and not boring) of well; PA1 b) selection of a mud system having a relatively high circulation rate to keep the hole clean and reduce filter cake build up; PA1 c) cleaning of well bore; PA1 d) final mud flush to clean filter cake from hole followed by scavenger slurry flush immediately prior to cementing; PA1 e) cementing of wellbore while casing is rotated and moved vertically and using a low shrinkage, pliable expandable cement; PA1 f) perforation of cement using low impact perforation techniques or casing cutting or slotting techniques, wherein the perforation interval does not exceed 10 meters in length and comprises a perforation density of about 20 shots/meter, with covering between 90.degree. and 120.degree. phasing.
Apparatus for the carrying out of the slurry injection process must meet several design criteria in order to dispose of a substantial volume of wastes at a high rate:
As well, the apparatus should be capable of operating on a generally continuous basis, and comprise an integrated system that is adapted to receive wastes, convert the wastes into an appropriate slurry, and discharge the slurry under pressure into a disposal well.
The operating parameters require equipment capable of injecting a relatively granular, highly viscous slurry at high rates and pressures. Preferably, the slurry formation and injection apparatus should provide the following:
An object of the invention is provide apparatus that addresses these requirements.
Monitoring of conditions within the target stratum serves two functions. First, it insures that the injection procedure is optimized for maximal injection speed and overall waste injection volume. Second, it provides evidence to regulatory agencies and other outside bodies that the injection process is being properly implemented and that the wastes are being confined within the target stratum. These goals may be furthered by monitoring and recording several variables in addition to those outlined above. In particular, slurry density, pressure, volume and composition should be monitored and recorded at all times. Alterations in large-scale permeability within the target stratum, excessive pressure build-up, abnormal fracture pressure, too-rapid pressure decay or other anomalous reservoir responses can be identified and analyzed to decide if these present problems for the continuation of the injection process in a particular well. It is an object of the invention to provide monitoring means that address these requirements.
In one example, the use of SFI permits permanent, low risk disposal of Non-Hazardous Oil Field Waste, comprising waste material and produced water. Waste generated at an oil field location can often be re-injected at fracture pressure through existing wells into the same subsurface formations from which the wastes originated. Another candidate for the SFI method is naturally occurring radioactive materials present in produced water, scale and sand from oil fields in many regions such as the Gulf Coast of the United States.